The present invention generally pertains to Code Division Multiple Access (CDMA) communications, also known as spread-spectrum communications. More particularly, the present invention pertains to a new system and method employing a new code sequence design for providing fast acquisition of a received spreading code phase in a CDMA communications system.
Recent advances in wireless communications have used spread spectrum modulation techniques to provide simultaneous communication by multiple users. Spread spectrum modulation refers to modulating a information signal with a spreading code signal; the spreading code signal being generated by a code generator where the period Tc of the spreading code is substantially less than the period of the information data bit or symbol signal. The code may modulate the carrier frequency upon which the information has been sent, called frequency-hopped spreading, or may directly modulate the signal by multiplying the spreading code with the information data signal, called direct-sequence (DS) spreading. Spread-spectrum modulation produces a signal with bandwidth substantially greater than that required to transmit the information signal. The original information is recovered at the receiver by synchronously demodulating and despreading the signal. The synchronous demodulator uses a reference signal to synchronize the despreading circuits to the input spread-spectrum modulated signal in order to recover the carrier and information signals. The reference signal may be a spreading code which is not modulated by an information signal. Such use of a synchronous spread-spectrum modulation and demodulation for wireless communication is described in U.S. Pat. No. 5,228,056 entitled SYNCHRONOUS SPREAD-SPECTRUM COMMUNICATIONS SYSTEM AND METHOD by Donald L. Schilling, which techniques are incorporated herein by reference.
One area in which spread-spectrum techniques are used is in the field of mobile cellular communications to provide personal communication services (PCS). Such systems desirably support large numbers of users, control Doppler shift and fade, and provide high speed digital data signals with low bit error rates. These systems employ a family of orthogonal or quasi-orthogonal spreading codes, with a pilot spreading code sequence synchronized to the family of codes. Each user is assigned one of the spreading codes as a spreading function. Related problems of such a system include: handling multipath fading effects. Solutions to such problems include diversity combining of multipath signals. Such problems associated with spread spectrum communications, and methods to increase capacity of a multiple access, spread-spectrum system are described in U.S. Pat. No. 4.901,307 entitled SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS by Gilhousen et al. which is incorporated herein by reference.
The problems associated with the prior art systems focus around reliable reception and synchronization of the receiver despreading circuits to the received signal. The presence of multipath fading introduces a particular problem with spread spectrum receivers in that a receiver must somehow track the multipath components to maintain code-phase lock of the receiver""s despreading means with the input signal. Prior art receivers generally track only one or two of the multipath signals, but this method may not be satisfactory because the combined group of low power multipath signal components may actually contain far more power than the one or two strongest multipath components. The prior art receivers track and combine the strongest components to maintain a predetermined Bit Error Rate (BER) of the receiver. Such a receiver is described, for example, in U.S. Pat. No. 5,109,390 entitled DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM by Gilhousen et al. A receiver that combines all multipath components, however, is able to maintain the desired BER with a signal power that is lower than that of prior art systems because more signal power is available to the receiver. Consequently, there is a need for a spread spectrum communication system employing a receiver that tracks substantially all of the multipath signal components, so that substantially all multipath signals may be combined in the receiver, and hence reduce the required transmit power of the signal for a given BER.
Providing quality telecommunication services to user groups which are classified as remote. Such as rural telephone systems and telephone systems in underdeveloped countries, has proved to be a challenge in recent years. These needs have been partially satisfied by wireless radio services, such as fixed or mobile frequency division multiplex (FDM), frequency division multiple access (FDMA), time division multiplex (TDM), time division multiple access (TDMA) systems, combination frequency and time division systems (FD/TDMA), and other land mobile radio systems. Usually, these remote services are faced with more potential users than can be supported simultaneously by their frequency or spectral bandwidth capacity.
The problems associated with the prior art systems focus around reliable reception and synchronization of the receiver despreading circuits to the received signal. Since spreading code sequences in a communications system which supports a relatively large number of users may be very long with a corresponding long code period, one particular problem associated with prior spread spectrum receivers is to rapidly determine the correct code phase of a received spread spectrum signal. One solution of fast acquisition of the correct spreading code phase is to form spreading code sequences with specific characteristics which a receiver can derive from a particular received code phase.
For example, prior art systems employ a method in which a code generator produces a pseudorandom code of length N, divides the code in half to generate two new codes with code period N/2, and multiplies the data with each code for transmission over an In-phase and Quadrature channel. The receiver only searches for the occurrence of the short code period on the I or Q channel. The advantage of the system is that the number of users supportable with codes of length N can be transmitted with a bandwidth necessary to support codes of length N/2. Such a system is described in U.S. Pat. No. 5,442,662 entitled CODE-DIVISION MULTIPLE-ACCESS COMMUNICATIONS SYSTEM PROVIDING ENHANCED CAPACITY WITHIN LIMITED BANDWIDTH to Fakasawa et al. with is incorporated herein by reference.
Another method and apparatus for producing a composite code for fast acquisition in a CDMA system may employ a code that is made to appear more complex by the use of one or more masking codes. The composite code generator comprises a plurality of component code generators. The composite codes are used to modulate in-phase and quadrature channels. A receiver has enhanced speed of acquisition because of the shorter time needed to search for composite codes in the quadrature channel, and the plurality of component codes of the in-phase channel are derived from the codes used in the quadrature channel. Such a system is described in U.S. Pat. No. 5,022,049 entitled MULTIPLE ACCESS CODE ACQUISITION SYSTEM to Abrahamson et al. which is incorporated herein by reference.
In related CDMA systems, a two-tier ciphering method ensures security by cycling code masks. A pseudorandomly generated code key is used to select one of a plurality of scrambling masks. A variant of this method uses orthogonal code hopping or random code hopping. A CDMA system can be viewed as encoding an information signal into blocks of L code symbols, and each block is then encoded with a scrambling mask of length L. A system of this type is described in U.S. Pat. No. 5,353,352, entitled CALLING CHANNEL IN CDMA COMMUNICATIONS SYSTEM to Dent et al. which is incorporated herein by reference.
Rapid acquisition of the correct code phase by a spread-spectrum receiver is improved by designing spreading codes which are faster to detect. The present embodiment of the invention includes a new method of generating code sequences that have rapid acquisition properties by using one or more of the following methods. First, a long code may be constructed from two or more short codes. The new implementation uses many code sequences, one or more of which are rapid acquisition sequences of length L that have average acquisition phase searches r=log2L. Sequences with such properties are well known to those practiced in the art. The average number of acquisition test phases of the resulting long sequence is a multiple of r=log2L rather than half of the number of phases of the long sequence.
Second, a method of transmitting complex valued spreading code sequences (In-phase (I) and Quadrature (Q) sequences) in a pilot spreading code signal may be used rather than transmitting real valued sequences. Two or more separate code sequences may be transmitted over the complex channels. If the sequences have different phases, an acquisition may be done by acquisition circuits in parallel over the different code sequences when the relative phase shift between the two or more code channels is known. For example, one of two sequences may he sent on an In phase (I) channel while the other is sent on the Quadrature (Q) channel. To search the code sequences, the acquisition detection means searches the two channels, but begins the (Q) channel with an offset equal to one-half of the length of the spreading code sequence. With a code sequence length of N, the acquisition means starts the search at N/2 on the (Q) channel. The average number of tests to find acquisition is N/2 for a single code search, but searching the (I) and phase delayed (Q) channel in parallel reduces the average number of tests to N/4. The codes sent on each channel may be the same code, with the code phase in one channel being delayed with respect to the other channel, or they may be different code sequences.